The invention relates to a system or apparatus for energy generation or storage on an electrochemical basis, which includes at least one flow cell, each flow cell including two half-cells through which differently charged electrolyte liquids flow and which are separated by a membrane, at least one electrode being disposed in each of these half-cells, a tank being provided for each of the electrolyte liquids, and in particular a system in which the electrolyte liquid in the positive half-cell in the charged state contains tetravalent and pentavalent vanadium, sulphuric acid and optionally further additives.